FIELD OF THE INVENTION
The invention relates to an electrically heatable honeycomb body through which a fluid can flow, including a plurality of subregions being electrically separate from one another, particularly for use as a carrier body for heatable catalytic converters in internal combustion engines, wherein an electric current can flow through the honeycomb body over a winding path around insulating points from one subregion to another.
Electrically heatable honeycomb bodies are used as carrier bodies for catalytic converters in order to convert pollutants in combustion exhaust gases of internal combustion engines and they are employed especially in precatalytic converters. In view of the increasingly stringent exhaust gas regulations worldwide for internal combustion engines, it is important for the most complete possible conversion of the exhaust gases to take place even during the cold starting phase. Heatable precatalytic converters, which are heated shortly before and/after the engine is started, are used for that purpose. That assures that the initially still relatively cold exhaust gases will be catalytically converted quickly. Electrically heatable honeycomb bodies are also employed in air heaters and the like.
Such electrically heatable honeycomb bodies are disclosed in International Application WO 89/10471, corresponding to U.S. Pat. No. 5,322,672, and in International Application WO 92/02714, corresponding generally to U.S. application Ser. No. 08/031,953, filed Feb. 1, 1993, for instance. In order to attain a suitable electrical resistance in that case, the honeycomb body is electrically subdivided by gaps or electrically insulating intermediate layers in such a way that at least one electrical current path having the desired resistance is produced.
European Application No. 0 452 125 A2, an electrically heatable honeycomb body is known that is prefereably extruded from a powdered metal slurry and is provided with electrodes on its outer surface. Structures such as slits are made in the honeycomb body and force the electrical current in the honeycomb body to take a winding path. The electric current is turned around or diverted at the ends of the slits. For a given applied voltage, an increased current density and therefore pronounced local heating preferentially occurs at such turning points at the ends of the slits. The honeycomb body can suffer damage at such "hot spots". That not only shortens the service life and causes damage to the honeycomb body, but also means that the honeycomb body is not heated homogeneously either, resulting in an unevenly distributed catalytic action within the honeycomb body. The overall result is that optimal conditions for catalytic operation do not prevail universally inside the honeycomb body for a fluid flow distributed through the entire honeycomb body. The slit honeycomb body is unstable in the presence of vibrational strains between its only two opposed fixed points (which at the same time are current lead-throughs). It must be supported against the housing by means of resilient and electrically insulating "fiber mats". The fibers mats are not stable or elastic over the long term at the temperatures that prevail in the exhaust gas system. The fibers are abraded by the strain. The honeycomb packet, with an elongated length of up to more than 300 mm, is disposed in such a way that it can vibrate freely between the two fixed points. The low natural frequency of that kind of configuration is in the range of the vibrational excitations that occur in the exhaust gas system. The insulating mats must then not only serve to provide durable elastic suspension but also must take on a vibration damping function without wear. Swelling mats of the kind used between a ceramic honeycomb and a metal jacket function fundamentally differently. They swell with the temperature. Swelling mats between metal structures and in the presence of such differential expansions as in that case would have to be dipped or formed by upsetting. Mica, for instance, can therefore not be used.